Expression of epidermis-specific antigens during embryogenesis of the ascidian, Halocynthia roretzi

A chordate species lacking Nodal utilizes calcium oscillation and Bmp for left-right patterning

Larvaceans are chordates with a tadpole-like morphology. In contrast to most chordates of which early embryonic morphology is bilaterally symmetric and the left-right (L-R) axis is specified by the Nodal pathway later on, invariant L-R asymmetry emerges in four-cell embryos of larvaceans. The asymmetric cell arrangements exist through development of the tailbud. The tail thus twists 90° in a counterclockwise direction relative to the trunk, and the tail nerve cord localizes on the left side. Here, we demonstrate that larvacean embryos have nonconventional L-R asymmetries: 1) L- and R-cells of the two-cell embryo had remarkably asymmetric cell fates; 2) Ca²⁺ oscillation occurred through embryogenesis; 3) Nodal, an evolutionarily conserved left-determining gene, was absent in the genome; and 4) bone morphogenetic protein gene (Bmp) homolog Bmp.a showed right-sided expression in the tailbud and larvae. We also showed that Ca²⁺ oscillation is required for Bmp.a expression, and that BMP signaling suppresses ectopic expression of neural genes. These results indicate that there is a chordate species lacking Nodal that utilizes Ca²⁺ oscillation and Bmp.a for embryonic L-R patterning. The right-side Bmp.a expression may have arisen via cooption of conventional BMP signaling in order to restrict neural gene expression on the left side.

Control of Pem protein level by localized maternal factors for transcriptional regulation in the germline of the ascidian, Halocynthia roretzi

Localized maternal mRNAs play important roles in embryogenesis, e.g. the establishment of embryonic axes and the developmental cell fate specification, in various animal species. In ascidians, a group of maternal mRNAs, called postplasmic/PEM RNAs, is localized to a subcellular structure, called the Centrosome-Attracting Body (CAB), which contains the ascidian germ plasm, and is inherited by the germline cells during embryogenesis. Posterior end mark (Pem), a postplasmic/PEM RNAs member, represses somatic gene expression in the germline during cleavage stages by inhibition of RNA polymerase II activity. However, the functions of other postplasmic/ PEM RNAs members in germline formation are largely unknown. In this study, we analyzed the functions of two postplasmic/PEM RNAs, Popk-1 and Zf-1, in transcriptional regulation in the germline cells. We show that Popk-1 contributes to transcriptional quiescence by controlling the size of the CAB and amount of Pem protein translated at the CAB. Our studies also indicated that zygotic expression of a germline gene starts around the onset of gastrulation and that the decrease of Pem protein is necessary and sufficient for the zygotic germline gene expression. Finally, further studies showed that the decrease of the Pem protein level is facilitated by Zf-1. Taken together, we propose that postplasmic/PEM RNAs such as Popk-1 and Zf-1 control the protein level of the transcriptional repressor Pem and regulate its transcriptional state in the ascidian germline.

Differential gene expression along the animal-vegetal axis in the ascidian embryo is maintained by a dual functional protein Foxd

In many animal embryos, a specific gene expression pattern is established along the animal-vegetal axis soon after zygotic transcription begins. In the embryo of the ascidian Ciona intestinalis, soon after the division that separates animal and vegetal hemispheres into distinct blastomeres, maternal Gata.a and β-catenin activate specific genes in the animal and vegetal blastomeres, respectively. On the basis of these initial distinct gene expression patterns, gene regulatory networks promote animal cells to become ectodermal tissues and vegetal cells to become endomesodermal tissues and a part of the nerve cord. In the vegetal hemisphere, β-catenin directly activates Foxd, an essential transcription factor gene for specifying endomesodermal fates. In the present study, we found that Foxd also represses the expression of genes that are activated specifically in the animal hemisphere, including Dmrt1, Prdm1-r.a (Bz1), Prdm1-r.b (Bz2), and Otx. A reporter assay showed that Dmrt1 expression was directly repressed by Foxd, and a chromatin immunoprecipitation assay showed that Foxd was bound to the upstream regions of Dmrt1, Prdm1-r.a, Prdm1-r.b, and Otx. Thus, Foxd has a dual function of activating specific gene expression in the vegetal hemisphere and of repressing the expression of genes that are normally expressed in the animal hemisphere. This dual function stabilizes the initial patterning along the animal-vegetal axis by β-catenin and Gata.a.

In embryogenesis of most animals, a specific gene expression pattern is established along the animal-vegetal axis first. In the embryo of the ascidian Ciona intestinalis, the activity of the maternal factor Gata.a is suppressed by β-catenin, which is active only in the vegetal hemisphere, and thereby these two factors activate specific genes in the animal and vegetal blastomeres, respectively. We found that a gene encoding a transcription factor, Foxd, which is a direct target of β-catenin, works as a promoter for endomesodermal fate and an inhibitor for ectodermal fate. In the ascidian embryo, the animal-vegetal axis initially established by the maternal factors is not stable enough for subsequent developmental processes, and needs to be maintained by Foxd. Thus, the animal hemisphere fate is suppressed first by the maternal factor β-catenin, and then by Foxd, which is activated by β-catenin. The primary embryonic axis is not stable initially, and stabilized by a transcription factor, which is expressed differentially along the axis.

Macho-1 regulates unequal cell divisions independently of its function as a muscle determinant

The anterior-posterior (A-P) axis in ascidian embryos is established through the posteriorizing activities of a localized egg region known as the posterior vegetal cortex/cytoplasm (PVC). Here we describe a novel function of macho-1, a maternally-localized muscle determinant, in establishment of the A-P axis in the Halocynthia roretzi embryo. Macho-1, in addition to its known function in the formation of posterior tissue such as muscle and mesenchyme, and suppression of the anterior-derived notochord fate, acts independently of its transcriptional activity as a regulator of posterior-specific unequal cell divisions, in cooperation with beta-catenin. Our results suggest that macho-1 and beta-catenin regulate the formation of a microtubule bundle that shortens and pulls the centrosome toward a sub-cellular cortical structure known as centrosome-attracting body (CAB), which is located at the posterior pole of the embryo during unequal cell divisions, and act upstream of PEM, a recently-identified regulator of unequal cell divisions. We also present data that suggest that PEM localization to the CAB may not be required for unequal cleavage regulation. The present study provides an important and novel insight into the role of the zinc-finger-containing transcription factor and indicates that it constitutes a major part of the PVC activity.

Nuclear accumulation of β-catenin and transcription of downstream genes are regulated by zygotic Wnt5α and maternal Dsh in ascidian embryos

Nuclear beta-catenin plays crucial roles in the establishment of the embryonic axis and formation of mesendoderm tissues in ascidians and other animals. However, the cue responsible for nuclear accumulation of beta-catenin in the vegetal hemisphere is still unknown in ascidians. Here, we investigated the roles of Wnt5alpha and Dsh in the nuclear accumulation of beta-catenin and activation of its downstream genes in the ascidian Halocynthia roretzi. Wnt5alpha knockdown embryos lost nuclear accumulation of beta-catenin at the 64-cell stage but not at the 32-cell stage, and expression of Hr-lim, one of the targets of beta-catenin, was impaired in the anterior region of the embryo. Zygotic Wnt5alpha expression in the anterior-vegetal blastomeres was primarily responsible for these defects. Dsh knockdown showed no effect on nuclear localization of beta-catenin, but inhibited Hr-lim expression in the posterior region. These results suggest that maintenance of nuclear Hr-beta-catenin after the 64-cell stage is regulated by zygotic Hr-Wnt5alpha, and that expression of its target genes is modulated by both Hr-Wnt5alpha and Hr-Dsh. Our results also highlight the importance of nuclear accumulation of beta-catenin up to the 32-cell stage through a still unclarified mechanism.

The functional analysis of Type I postplasmic/PEM mRNAs in embryos of the ascidian Halocynthia roretzi

Maternal factors, such as a muscle determinant macho-1 mRNA that is localized to the posterior-vegetal cortex (PVC) of fertilized ascidian eggs, are crucial for embryonic axis formation and cell fate specification. Maternal mRNAs that show an identical posterior localization pattern to that of macho-1 in eggs and embryos are called Type I postplasmic/PEM mRNAs. We investigated the functions of five of the nine Type I mRNAs so far known in Halocynthia roretzi: Hr-Wnt-5, Hr-GLUT, Hr-PEM3, Hr-PEN1, and Hr-PEN2. Suppression of their functions with specific antisense morpholino oligonucleotides (MOs) had effects on the formation of various tissues: Hr-Wnt-5 on notochord, muscle, and mesenchyme, although zygotic function of Hr-Wnt-5 is responsible for notochord formation; Hr-GLUT on notochord, mesenchyme, and endoderm; and Hr-PEN2 on muscle, mesenchyme, and endoderm. On the other hand, Hr-PEM3 and Hr-PEN1 MOs seemed to have no effect. We conclude that the functions of at least some localized maternal Type I postplasmic/PEM mRNAs are necessary for early embryonic patterning in ascidians.

An Ets transcription factor, HrEts, is target of FGF signaling and involved in induction of notochord, mesenchyme, and brain in ascidian embryos

In ascidian embryos, a fibroblast growth factor (FGF) signal induces notochord, mesenchyme, and brain formation. Although a conserved Ras/MAPK pathway is known to be involved in this signaling, the target transcription factor of this signaling cascade has remained unknown. We have isolated HrEts, an ascidian homolog of vertebrate Ets1 and Ets2, to elucidate the transcription factor involved in the FGF signaling pathway in embryos of the ascidian Halocynthia roretzi. Maternal mRNA of HrEts was detected throughout the entire egg cytoplasm and early embryos. Its zygotic expression started in several tissues, including the notochord and neural plate. Overexpression of HrEts mRNA did not affect the general organization of the tadpoles, but resulted in formation of excess sensory pigment cells. In contrast, suppression of HrEts function by morpholino antisense oligonucleotide resulted in severe abnormalities, similar to those of embryos in which the FGF signaling pathway was inhibited. Notochord-specific Brachyury expression at cleavage stage and notochord differentiation at the tailbud stage were abrogated. Formation of mesenchyme cells was also suppressed, and the mesenchyme precursors assumed muscle fate. In addition, expression of Otx in brain-lineage blastomeres was specifically suppressed. These results suggest that an Ets transcription factor, HrEts, is involved in signal transduction of FGF commonly in notochord, mesenchyme, and brain induction in ascidian embryos.

Role of the FGF and MEK signaling pathway in the ascidian embryo

In the ascidian embryo, a fibroblast growth factor (FGF)-like signal from presumptive endoderm blastomeres between the 32-cell and early 64-cell stages induces the formation of notochord and mesenchyme cells. However, it has not been known whether endogenous FGF signaling is involved in the process. Here it is shown that 64-cell embryos exhibit a marked increase in endogenous extracellular signal-regulated kinase (ERK/MAPK) activity. The increase in ERK activity was reduced by treatment with an FGF receptor 1 inhibitor, SU5402, and a MEK (ERK kinase/MAPKK) inhibitor, U0126. Both drugs blocked the formation of notochord and mesenchyme when embryos were treated at the 32-cell stage, but not at the 2- or 110-cell stages. The dominant-negative form of Ras also suppressed notochord and mesenchyme formation. Both inhibitors suppressed induction by exogenous basic FGF. These results suggest that the FGF signaling cascade is indeed necessary for the formation of notochord and mesenchyme cells during ascidian embryogenesis. It is also shown that FGF signaling is required for formation of the secondary notochord, secondary muscle and neural tissues, and at least ERK activity is necessary for the formation of trunk lateral cells and posterior endoderm. Therefore, FGF and MEK signaling are required for the formation of various tissues in the ascidian embryo.

The BMP/CHORDIN antagonism controls sensory pigment cell specification and differentiation in the ascidian embryo

We have investigated the role of the bone morphogenetic protein (BMP) pathway during neural tissue formation in the ascidian embryo. The orthologue of the BMP antagonist, chordin, was isolated from the ascidian Halocynthia roretzi. While both the expression pattern and the phenotype observed by overexpressing chordin or BMPb (the dpp-subclass BMP) do not suggest a role for these factors in neural induction, BMP/CHORDIN antagonism was found to affect neural patterning. Overexpression of BMPb induced ectopic sensory pigment cells in the brain lineages that do not normally form pigment cells and suppressed pressure organ formation within the brain. Reciprocally, overexpressing chordin suppressed pigment cell formation and induced ectopic pressure organ. We show that pigment cell formation occurs in three steps. (1) During cleavage stages ectodermal cells are neuralized by a vegetal signal that can be substituted by bFGF. (2) At the early gastrula stage, BMPb secreted from the lateral nerve cord blastomeres induces those neuralized blastomeres in close proximity to adopt a pigment cell fate. (3) At the tailbud stage, among these pigment cell precursors, BMPb induces the differentiation of specifically the anterior type of pigment cell, the otolith; while posteriorly, CHORDIN suppresses BMP activity and allows ocellus differentiation.

Nuclear plasticity and timing mechanisms of the initiation of alkaline phosphatase expression in cytoplasm-transferred blastomeres of ascidians

Egg cytoplasm containing endoderm determinants was transferred to presumptive-muscle or presumptive-epidermis blastomeres isolated from cleavage-stage embryos of the ascidian Halocynthia roretzi. We investigated three aspects of the expression of endoderm-specific alkaline phosphatase (ALP) activity. First, we examined whether ectopic ALP expression, an indication of ectopic endoderm formation, was promoted in cytoplasm-transferred blastomeres isolated at late-cleavage stage. The results showed that the cell fate was converted by the introduced cytoplasm, even in recipient blastomeres in which the cell fate was already restricted to muscle or epidermis, and in those where expression of the muscle- or epidermis-specific genes was already initiated. Next, we examined the formation of endoderm and other tissue in embryos by double staining for ALP and muscle- or epidermis-specific marker. Regions positive for ALP and positive for muscle or epidermis marker were mutually exclusive. These results suggested that muscle- or epidermis-specific genes that were already expressed in the recipient blastomeres were down-regulated in ectopically forming endoderm cells. This is evidence for nuclear plasticity during ascidian embryogenesis. In the last series of experiments, we investigated the timing of the appearance of ALP activity in cytoplasm-transferred embryos. In the partial embryos that were derived from various combination of recipient blastomeres and donor cytoplasm obtained from various staged eggs and embryos, the timing seemed to coincide with the time that starts when cell fusion for cytoplasmic transfer was done. Therefore, the clock that determines the timing of the initiation of ALP expression is likely to start at the moment of cell fusion. Several possible hypotheses for the timing mechanism are discussed.

Determinants of cell and positional fate in ascidian embryos

Ascidians have played a major role in studies to understand the function of cytoplasmic determinants in animal development. Special qualities, including eggs with colored cytoplasmic regions, an invariant cleavage pattern and cell lineage, embryos with low cell numbers, larvae with typical chordate features and only six different tissues, rapid development, and a small genome, combine to make these animals a unique system for studying cytoplasmic determinants. There is evidence for determinants that specify the cleavage pattern; the differentiation of epidermal, endodermal, and muscle cells; and cell movements associated with gastrulation. The muscle determinants appear to be modified in concert with tail and muscle regression in species that have evolved an anural, or tailless, larva. Several lines of evidence suggest that determinants may be localized maternal mRNAs, which encode transcription factors or signal transduction components responsible for initiating differential gene activity. Different approaches and strategies are being used to isolate and characterize the function of these localized maternal mRNAs.

Ascidian tail formation requires caudal function

Although the tail is one of the major characteristics of animals of the phylum Chordata, evolutionary aspects of the molecular mechanisms involved in its formation are not clear. To obtain insights into these issues, we have isolated and investigated the caudal gene of an ascidian, one of the lower animal groups among chordates. Ascidian caudal is expressed from the midgastrula stage onward in the lateral walls of the posterior neural tube cell lineage and also in the posterior epidermal cells from the neurula stage. Thus, ascidian caudal expression is restricted to the ectoderm of a tail-forming region throughout embryogenesis. Suppression of caudal function by an antisense oligonucleotide or a dominant negative construct caused inhibition of the cell movement required for tail formation. Overexpression of wild-type caudal mRNA in an ascidian animal cap, an animal half explant prepared at the eight-cell stage, caused elongation of the cap. Furthermore, Xenopus embryos injected with dominant negative ascidian caudal exhibited defects in elongation, suggesting a conserved caudal function among chordates. These results indicate that caudal function is required for chordate tail formation and may play a key role in its evolution.

Ion channels and early development of neural cells

In this review we underscore the merits of using voltage-dependent ion channels as markers for neuronal differentiation from the early stages of uncommitted embryonic blastomeres. Furthermore, a fairly large part of the review is devoted to the descriptions of the establishment of a simple model system for neural induction derived from the cleavage-arrested eight-cell ascidian embryo by pairing a single ectodermal with a single vegetal blastomere as a competent and an inducer cell, respectively. The descriptions are focused particularly on the early developmental processes of various ion channels in neuronal and other excitable membranes observed in this extraordinarily simple system, and we compare these results with those in other significant and definable systems for neural differentiation. It is stressed that this simple system, for which most of the electronic and optical methods and various injection experiments are applicable, may be useful for future molecular physiological studies on the intracellular process of differentiation of the early embryonic cells. We have also highlighted the importance of suppressive mechanisms for cellular differentiation from the experimental results, such as epidermal commitment of the cleavage-arrested one-cell Halocynthia embryos or suppression of epidermal-specific transcription of inward rectifier channels by neural induction signals. It was suggested that reciprocal suppressive mechanisms at the transcriptional level may be one of the key processes for cellular differentiation, by which exclusivity of cell types is maintained.

Cell fate specification by localized cytoplasmic determinants and cell interactions in ascidian embryos

Tadpole larvae of ascidians show the basic body plan of chordates. An ascidian larva consists of only a few types of cells and has a relatively small number of cells. Cell lineages are invariant among individuals and have been described in detail. These advantages facilitate the analysis of how the fate of each blastomere becomes specified during development. Over a century of research on ascidian embryogenesis has uncovered many interesting features concerning cellular mechanisms responsible for the fate specification. During embryogenesis, the developmental fate of a blastomere is specified by one of three different mechanisms: localized maternal cytoplasmic determinants, inductive interactions, or lateral inhibition in an equivalence cell group.

Ras is an essential component for notochord formation during ascidian embryogenesis

In ascidian embryos, inductive interactions are necessary for the fate specification of notochord cells. Previous studies have shown that notochord induction occurs at the 32-cell stage and that basic fibroblast growth factor (bFGF) has notochord-inducing activity in ascidian embryos. In vertebrate, it is known that bFGF receptors have tyrosine kinase domain and the signaling pathway is mediated by the small-GTP binding protein, Ras. To study the role of Ras in ascidian embryos, we injected dominant negative Ras (RasN17) into fertilized eggs. RasN17 inhibited the formation of notochord, suggesting that the Ras signaling pathway is involved in signal transduction in the induction of notochord cells. When the presumptive-notochord (A6.2) blastomere was co-isolated with the inducer (A6.1) blastomere and then RasN17 was injected into the A6.2 blastomere, notochord differentiation was suppressed. The presumptive-notochord blastomeres injected with RasN17 were treated with bFGF. Many of them failed to develop notochord-specific features. Next, we examined the effect of injecting constitutively active Ras (RasV12) into the A6.2 blastomeres. However, microinjection of RasV12 into these cells did not bypass notochord induction. These results suggest that the Ras signaling pathway is essential for the formation of notochord and that another signaling pathway also must be activated simultaneously in notochord formation during ascidian embryogenesis.

Distinct neuronal lineages of the ascidian embryo revealed by expression of a sodium channel gene

The ascidian larva contains tubular neural tissue, one of the prominent anatomical features of the chordates. The cell-cleavage pattern and cell maps of the nervous system have been described in the ascidian larva in great detail. Cell types in the neural tube, however, have not yet been defined due to the lack of a suitable molecular marker. In the present work, we identified neuronal cells in the caudal neural tube of the Halocynthia embryo by utilizing a voltage-gated Na+ channel gene, TuNa I, as a molecular marker. Microinjection of a lineage tracer revealed that TuNa I-positive neurons in the brain and in the trunk epidermis are derived from the a-line of the eight-cell embryo, which includes cell fates to epidermal and neural tissue. On the other hand, TuNa I-positive cells in the more caudal part of the neural tissue were not stained by microinjection into the a-line. These neurons are derived from the A-line, which contains fates of notochord and muscle, but not of epidermis. Electron microscopic observation confirmed that A-line-derived neurons consist of motor neurons innervating the dorsal and ventral muscle cells. Isolated A-line blastomeres have active membrane excitability distinct from those of the a-line-derived neuronal cells after culture under cleavage arrest, suggesting that the A-line gives rise to a neuronal cell distinct from that of the a-lineage. TuNa I expression in the a-line requires signals from another cell lineage, whereas that in the A-line occurs without tight cell contact. Thus, there are at least two distinct neuronal lineages with distinct cellular behaviors in the ascidian larva: the a-line gives rise to numerous neuronal cells, including sensory cells, controlled by a mechanism similar to vertebrate neural induction, whereas A-line cells give rise to motor neurons and ependymal cells in the caudal neural tube that develop in close association with the notochord or muscle lineage, but not with the epidermal lineage.

Analysis of the temporal expression of endoderm-specific alkaline phosphatase during development of the ascidian Halocynthia roretzi

During embryogenesis of ascidians, endoderm cells initiate certain processes associated with differentiation and produce a tissue-specific enzyme, alkaline phosphatase (ALP). ALP has been used as a histochemical marker of endoderm differentiation. In the present study, the temporal profile of ALP expression during embryogenesis was investigated. In Halocynthia roretzi, endoderm-specific ALP is a membrane bound protein and is distinguishable from maternal cytoplasmic ALP by molecular mass. The activity of endodermal ALP first appeared at the early tail-bud stage. Treatment of developing embryos with inhibitors of translation and transcription was started at various stages. The results suggested that the synthesis of endodermal ALP protein started at the early tail-bud stage, and that the transcription of mRNA was initiated in the gastrula. In other ascidians, Ciona and Styela, it has been suggested that a significant amount of maternal ALP mRNA exists in eggs. The present study revealed that there are significant species differences in ALP expression during ascidian embryogenesis.

Vegetal egg cytoplasm promotes gastrulation and is responsible for specification of vegetal blastomeres in embryos of the ascidian Halocynthia roretzi

An animal-vegetal axis exists in the unfertilized eggs of the ascidian Halocynthia roretzi. The first phase of ooplasmic segregation brings the egg cortex to the vegetal pole very soon after fertilization. In the present study, when 5–8% of the egg cytoplasm in the vegetal pole region was removed between the first and second phase of segregation, most embryos exhibited failure of gastrulation, as reported previously in Styela by Bates and Jeffery (Dev. Biol, 124, 65–76, 1987). The embryos that were deficient in vegetal pole cytoplasm (VC-deficient embryos) developed into permanent blastulae. They consisted for the most part of epidermal cells and most lacked the derivatives of vegetal blastomeres, such as endoderm, muscle and notochord. Removal of cytoplasm from other regions did not affect embryogenesis. The cleavage of the VC-deficient embryos not only exhibited radial symmetry along the animal-vegetal axis but the pattern of the cleavage was also identical in the animal and vegetal hemispheres. Examination of the developmental fates of early blastomeres of VC-deficient embryos revealed that the vegetal blastomeres had assumed the fate of animal cells. These results suggested that the VC-deficient embryos had been totally animalized. When vegetal pole cytoplasm was transplanted to the animal pole or equatorial position of VC-deficient eggs, gastrulation occurred, starting at the site of the transplantation and tissues derived from vegetal blastomeres formed. Therefore, it appears that vegetal pole cytoplasm specifies the site of gastrulation and the cytoplasm is responsible for the specification of vegetal blastomeres. It is suggested that during the second phase of ooplasmic segregation, cytoplasmic factors responsible for gastrulation spread throughout the entire vegetal hemisphere.

Chasing tails in ascidians: developmental insights into the origin and evolution of chordates

The ascidian tadpole larva is regarded as a prototype of the ancestral chordate. Here we consider recent studies on the development of the tadpole larva that provide new insights into chordate origins and evolution. The notochord of ascidian larvae and vertebrates appear to be homologous structures based on their induction by endoderm and expression of the Brachyury (T) gene. The muscle cells of ascidian larvae also appear homologous to those of vertebrates based on their expression of bHLH myogenic and muscle-type actin genes, although they are specified by cytoplasmic determinants localized in the egg as well as embryonic induction. Studies of the tailless larvae of anural ascidians have resulted in the identification of Manx, a gene that may control tail development and evolution. These and other results support the ascidian tadpole prototype for the ancestral chordate.

Evolution of alternate modes of development in ascidians

Ascidians have evolved alternate modes of development in which the conventional tadpole larva is remodeled or eliminated. Adultation, the precocious development of adult features in the larval head, is caused by superimposing the larval and adult differentiation programs. Caudalization, the addition of muscle cells to the larval tail, is caused by enhancing muscle induction or increasing the number of muscle cell divisions before terminal differentiation. Adultation and caudalization are correlated with increased egg size, suggesting dependence on maternal processes. Anural development, the elimination of the larval stage, is caused by maternal and zygotic events resulting in abbreviation and deletion of larval developmental programs. An example of a maternal change in anural species is the modification of the egg cytoskeleton during oogenesis, whereas a zygotic change may involve altered cell interactions during embryogenesis. Interspecific hybridization experiments suggest that some aspects of anural development may be caused by loss-of-function mutations. The dissociation of developmental programs is a key process in changing the mode of development in ascidians.

Developmental potential for tissue differentiation of fully dissociated cells of the ascidian embryo

Initially, each tissue-progenitor blastomere of embryos of the ascidian Halocynthia was identified and isolated manually at the 110-cell (late-blastula) stage, the time at which most of the blastomeres have assumed a particular fate, such that each gives rise to a single type of tissue. The isolates were allowed to develop as partial embryos, then tissue differentiation was examined by monitoring the expression of specific molecular markers for differentiation of epidermis, endoderm, muscle and notochord. Essentially, all of the precursor blastomeres of these four kinds of tissue expressed the appropriate features of tissue differentiation in isolation, indicating that determination is already complete in most of the blastomeres by the 110-cell stage. Next, in order to evaluate the absolute capacity of cells for autonomous development, embryos were maintained continuously in a dissociated state from the first cleavage to the 110-cell stage, then the cells were allowed to develop into partial embryos. Tissue differentiation in the partial embryos was examined. The results showed the striking autonomy of the processes of segregation of developmental potential, as well as the autonomy of the processes of expression of differentiated phenotypes, namely those of epidermis and endoderm. Autonomous muscle differentiation was also observed; however, excess formation of "muscle" partial embryos occurred. The hypothesis that fate determination is mediated by localized maternal information in the egg cytoplasm is supported by the evidence of development of these tissues. By contrast, no evidence of notochord differentiation was observed in the partial embryos.

Expression of an antigen specific for trunk lateral cells in quarter embryos of the ascidian, Halocynthia roretzi

Cell-lineage analysis has demonstrated that a pair of the right and left A7.6 cells of a 64-cell embryo of the ascidian Halocynthia roretzi, descendants of A4.1 cells of an 8-cell embryo, give rise to trunk lateral cells (TLCs). In this study, in order to investigate cellular mechanisms involved in the specification of TLCs, we have examined the expression of a TLC-specific antigen in cleavage-arrested embryos and in quarter partial embryos. Although cleavage arrest of embryos by treatment with cytochalasin B at early stages, prior to and including the 16-cell stage, inhibited expression of the TLC-specific antigen, embryos arrested at the 32-cell stage and at later stages developed the antigen. The only blastomeres exhibiting expression of the antigen were the presumptive TLCs, as predicted by cell-lineage assignments. When the developmental potential of quarter embryos that originated from four isolated blastomere-pairs (a4.2, b4.2, A4.1, and B4.1 pairs) of an 8-cell embryo was examined, the A4.1 quarter embryos, which are developmentally fated to give rise to TLCs, rarely showed evidence of expression of the antigen. Expression of the antigen was not observed in a4.2 and b4.2 quarter embryos, which are not associated with the TLC fate. By contrast, expression of the antigen was detected in about a half of the B4.1 quarter embryos which are also not associated with the TLC fate. These results are discussed with reference to the relationship between TLCs and mesenchyme cells.

Ultraviolet irradiation during ooplasmic segregation prevents gastrulation, sensory cell induction, and axis formation in the ascidian embryo

The effect of ultraviolet (uv) light on embryonic development was examined in the ascidian Styela clava. uv irradiation (3.0 x 10(-3) J mm-2) of the entire surface of fertilized eggs during ooplasmic segregation prevented gastrulation, sensory cell induction, and embryonic axis formation. The uv-irradiated embryos completed ooplasmic segregation and cleaved normally, but vegetal blastomeres did not invaginate at the beginning of gastrulation, sensory cells in the larval brain did not develop tyrosinase or melanin pigment, and the larval tail did not develop. Endoderm, epidermis, and muscle cells differentiated in the uv-irradiated embryos, however, as evidenced by expression of endodermal alkaline phosphatase (AP), an epidermal-specific antigen, and alpha-actin, myosin heavy chain, and acetylcholinesterase (AChE) in muscle cells. Higher doses of uv light (6.0-9.0 x 10(-3) J mm-2) suppressed expression of the epidermal antigen and muscle cell markers, whereas the development of endodermal AP was insensitive. Irradiation at various times between fertilization and the 16-cell stage revealed that gastrulation, sensory cell differentiation, and axis formation are sensitive to uv light only during ooplasmic segregation. Irradiation of restricted regions of the zygote during ooplasmic segregation showed that the uv-sensitive components are localized in the vegetal hemisphere. The absorption characteristics of the uv-sensitive components suggest that they are nucleic acids. The results show that uv-sensitive components that specify gastrulation, sensory cell induction, and embryonic axis formation are localized in the vegetal hemisphere of Styela eggs.

Differentiation of membrane excitability in isolated cleavage-arrested blastomeres from early ascidian embryos

1. Differentiation of excitable cells was studied electrophysiologically and histochemically in cleavage-arrested blastomeres isolated from early ascidian embryos. Blastomeres were isolated at the 4- or 8-cell stage, and cultured in sea water containing cytochalasin B until the time of hatching of control larvae. Electrical responses, immunoreactivity to epidermis-specific monoclonal antibody (2C5) and activity of muscle-specific acetylcholinesterase were examined. 2. All cleavage-arrested blastomeres isolated from an 8-cell embryo differentiated to elicit either muscular- or epidermal-type action potentials, but no neural-type action potentials were observed in these blastomeres. The anterior-animal and the posterior-animal blastomeres developed only epidermal-type action potentials, which involved expression of Ca2+ channels and immunoreactivity to 2C5. One-third of anterior-vegetal blastomeres developed epidermal-type action potentials which are mediated by Ca2+ channels though the immunoreactivity to 2C5 was absent. A majority of remaining blastomeres showed action potentials composed of Ca2+ currents and TEA-sensitive delayed K+ currents (type I response), and a few of them had fast transient K+ currents (A-currents) in addition (type II response). One-third of posterior-vegetal blastomeres developed epidermal-type action potentials without expression of the immunoreactivity to 2C5. The remainder differentiated into muscular-type cells, which expressed Ca2+ currents, TEA-sensitive and TEA-insensitive delayed K+ currents, and showed acetylcholinesterase activity. 3. Cleavage-arrested blastomeres isolated from a 4-cell embryo also differentiated into epidermal- or muscular-type cells, but not neural-type cells. The anterior blastomere, which is the parent cell of anterior-animal and anterior-vegetal blastomeres of an 8-cell embryo, developed epidermal-type, type I or type II responses, as was the case in the anterior-vegetal blastomere isolated from an 8-cell embryo. The posterior blastomere, which was the parent cell of posterior-animal and posterior-vegetal blastomeres of an 8-cell embryo, differentiated into either epidermal-type or muscular-type cells in terms of both membrane excitability and immunochemical reactivity. 4. Cleavage-arrested 1-cell embryos differentiated exclusively into epidermal-type cells in terms of membrane excitability and 2C5 immunoreactivity, even when the cytochalasin B concentration was decreased below 0.1 microgram/ml.(ABSTRACT TRUNCATED AT 400 WORDS)

An 83-kDa embryonic-type nuclear antigen is detected within the germinal vesicles of oocytes of the ascidianHalocynthia roretzi

Monoclonal antibodies were raised against germinal vesicles which were isolated from fully grown oocytes of the ascidianHalocynthia roretzi. Immunoblot analyses revealed that one of the antibodies, designated Hgv-2, recognized a single band with a molecular weight of about 83 kDa. The antibody, visualized by indirect immunohistochemistry, reacted only with the germinal vesicles of oocytes and did not react with test cells, follicle cells, and other somatic cells of the gonad. During embryogenesis the antigenicity was found in interphase nuclei of all embryonic cells. The antibody did not react with chromosomes or the mitotic apparatus. The antigenicity was retained by interphase nuclei of larval cells, but it disappeared from nuclei of juveniles about 7 days after metamorphosis.

Specification of notochord cells in the ascidian embryo analysed with a specific monoclonal antibody

Among 40 notochord cells of an ascidian tadpole larva, 32 notochord cells originate from the anterior-vegetal blastomeres (the A4.1 pair) of an 8-cell embryo and eight cells originate from the posterior-vegetal blastomeres (the B4.1 pair), but the animal blastomeres (the a4.2 and b4.2 pairs) are not engaged in the formation of the notochord. If four pairs of cells, separated from an 8-cell embryo, were allowed to develop into quarter embryos, expression of the notochord-specific antigen was evident in the A4.1 and B4.1 quarter embryos. Embryos, in which cytokinesis had been permanently blocked at the 8-cell and later stages with cytochalasin B, were found to develop the notochord-specific antigen only in the presumptive notochord cells. These findings suggest the developmental autonomy of presumptive notochord cells in the ascidian embryo.

A voltage-dependent chloride current linked to the cell cycle in ascidian embryos

A voltage-dependent chloride current has been found in early ascidian embryos that is a minor conductance in the oocyte and in interphase blastomeres but that increases transiently in amplitude by more than tenfold during each cell division. Repeated cycles in the density of this chloride current could be recorded for up to 6 hours (four cycles) in cleavage-arrested embryos, whether they were activated by sperm or calcium ionophore. These data suggest that there is a direct link between the cell cycle clock and the properties of this channel, a link that results in pronounced cyclical changes in the electrical properties of early blastomeres.

A simple "neural induction" model with two interacting cleavage-arrested ascidian blastomeres

A single anterior-animal blastomere, which includes the presumptive neural region in the eight-cell embryo of the Halocynthia, a protochordate, when dissociated, cleavage-arrested with cytochalasin B, and cultured in isolation, differentiated exclusively to epidermal type judging from membrane excitability and immunoreactivity. However, when the same blastomere was cultured in contact with a single anterior-vegetal blastomere, which includes the presumptive notochordal region, it displayed Na spikes and showed no expression of the epidermal antigen, suggesting that "neural induction" resulted in a single cell during the interaction with a single neighboring cell. This simple two-cell system can be used for further studies on the induction mechanism.